1. Field of the Invention
The present invention relates to an alignment pattern and a method of forming the same, and more particularly to an alignment pattern to be used for measuring a degree of misalignment in alignment process for patterning a metal interconnection film and a method of forming the same.
All of patents, patent applications, patent publications, scientific articles and the like, which will hereinafter be cited or identified in the present application, will, hereby, be incorporated by references in their entirety in order to describe more fully the state of the art, to which the present invention pertains.
2. Description of the Related Art
An alignment pattern is used for alignment to an existent pattern in order to form a new pattern without any displacement or misalignment with reference to the existent pattern.
There has been known a variety of alignment pattern with various shapes, for example, a box-mark which comprises a rectangle-shaped frame. A resist film is formed over the alignment pattern, so that the resist film has a surface which includes a level-difference or steps caused by the presence of the alignment pattern. The steps comprise slopes which provide boundaries between high-level and low-level regions of the surface of the resist film. The uniformity or symmetry in gradient of the slopes in connection with the single alignment pattern indicates the accuracy in alignment. The non-uniformity, variation or asymmetry in gradient of the slopes in connection with the single alignment pattern indicates the degree of miss-alignment.
One typical example of the alignment pattern may be a rectangular-flame-shaped pattern which will be referred to as a box-mark. The resist film is formed over the box-mark, so that the resist film has a surface which has steps which extend on a rectangular-flame-shaped region in plan view. The steps comprise sloped surfaces which provide boundaries between a rectangular-shaped inside flat surface and a surrounding outside flat surface which surrounds the rectangular-shaped inside flat surface. In one example, the rectangular-shaped inside flat surface has a lower level than the surrounding outside flat surface. The rectangular-shaped inside flat surface is communicated through the sloped surfaces as the steps to the surrounding outside flat surface.
The uniformity or symmetry of the distance in plan view between the rectangular-shaped inside flat surface and the surrounding outside flat surface indicates the accuracy of alignment. Namely, the uniformity or symmetry of the slope-distance of the steps or the sloped surfaces indicates the accuracy of alignment. The non-uniformity or asymmetry of the distance in plan view between the rectangular-shaped inside flat surface and the surrounding outside flat surface indicates the degree of miss-alignment. Namely, the non-uniformity or asymmetry of the slope-distance of the steps or the sloped surfaces indicates the degree of miss-alignment.
The alignment pattern of this box-mark may, for example, comprise the sloped surfaces which provide boundaries between a flat higher level surface of an inter-layer insulator and a flat lower level surface of a metal plug buried within an alignment hole formed in the inter-layer insulator, wherein the alignment hole has a rectangle-shape in plan view.
A contact hole for the box-mark is formed in the inter-layer insulator. A metal plug is buried within the contact hole. An etch-back is carried out to the surface of the buried metal plug to planarize the surface of the metal plug, whereby steps or sloped surfaces are formed between the planarized lower level surface of the metal plug and the planarized higher level surface of the inter-layer insulator. Namely, the contact hole has the steps or the sloped surfaces. An interconnection layer is formed over the inter-layer insulator and the contact hole, whereby the interconnection layer also has the steps or the sloped surfaces.
The high planarity can be obtained by a chemical mechanical polishing and a high temperature sputtering process of aluminum. In place of the etch-back process, the chemical mechanical polishing is used for planarization of the metal plug buried within the contact hole.
FIGS. 1A and 1B are fragmentary cross sectional elevation views of sequential steps involved in a conventional method for forming an alignment pattern.
As shown in FIG. 1A, a titanium silicide layer 2 is formed over a silicon substrate 1. An inter-layer insulator 3 comprising a boron phospho-silicate glass is formed over the titanium silicide layer 2. A scribe line for dividing a wafer comprises a diffusion layer. A contact hole is formed in a region of the diffusion layer. Not only a contact hole 4 having a diameter of not more than 0.5 micrometers is formed in the diffusion layer for an internal circuit but also an alignment hole 5 having a diameter of not less than 15 micrometers is also formed in an alignment region.
As shown in FIG. 1B, a tungsten plug 6 is formed over the, inter-layer insulator 3 and within the contact hole 4 and the alignment bole 5, whereby the contact hole 4 and the alignment hole 5 are completely filled with the tungsten plug 6. A chemical mechanical polishing is carried out to the tungsten plug 6 for planarization thereof, whereby the contact hole 4 and the alignment hole 5 have steps or level-difference. Namely, the tungsten plug 6 buried within each of the contact hole 4 and the alignment hole 5 has a planarized lower-level surface lower in level than the surface of the inter-layer insulator 3 and a sloped or stepped peripheral region, which surrounds the planarized lower-level surface and bounds the planarized lower-level surface from the surface of the inter-layer insulator 3. The planarized lower-level surface is bounded by the sloped or stepped peripheral region from the surface of the inter-layer insulator 3. The planarized lower-level surface of the tungsten plug 6 is surrounded by the sloped or stepped peripheral region of the upper surface of the tungsten plug 6. The tungsten plug 6 within the alignment hole 5 has steps which provide a level-difference “d” of about 50 nanometers. The sloped or stepped peripheral region serves as a box-mark. An aluminum interconnection layer 7 is formed over the surface of the inter-layer insulator 3 as well as over the planarized lower-level surfaces and the sloped or stepped peripheral regions of the tungsten plugs 6 within the contact hole 4 and the alignment hole 5.
FIG. 2 shows a conventional alignment pattern in a fragmentary cross sectional view and a fragmentary plan view as well as read out data about the steps.
The above-described processes shown in FIGS. 1A and 1B have been completed, thereby the aluminum interconnection layer 7 is formed over the surface of the inter-layer insulator 3 as well as over the planarized lower-level surfaces and the sloped or stepped peripheral regions of the tungsten plugs 6 within the contact hole 4 and the alignment hole 5. A resist film 8 is formed over the aluminum interconnection layer 7. The resist film 8 is then patterned with an alignment which is made by detecting the box-mark 9 which comprises the sloped surfaces or the steps in the alignment hole 5, for which purpose a scanning to the surface of the substrate is made in a direction shown in an arrow mark across a pair of opposite sides of the box-mark 9. As a result of the scanning, any level-differences can be detected as peak waveforms. The presence of the box-mark 9 which comprises the sloped surfaces or the steps forms further sloped surfaces or steps of the surface of the resist pattern 8. The further sloped surfaces or steps of the surface of the resist pattern 8 due to the presence of the box-mark 9 are represented to be peak waveforms “A” and “D”. The resist pattern 8 also has a square-shaped resist pattern hole 8-a, so that a part of the planarized lower-level surface of the tungsten plug 6 within the alignment hole 5 is exposed through the resist pattern hole 8-a. A periphery of the resist pattern hole 8-a is represented to be peak waveforms “B” and “C”.
If the planarization is carried out by the chemical mechanical polishing to the tungsten plug 6, then the level-difference or the sloped steps of the box-mark 9 is small even the alignment hole 5 has a larger diameter. The small level-difference of the box-mark 9 makes it difficult to detect the level-difference of the surface of the resist pattern 8 for the following reasons.
The chemical mechanical polishing to the tungsten plug 6 reduces a plug-loss of the tungsten plug 6, thereby reducing the level-difference between or the sloped steps as boundaries between the planarized lower-level surface of the tungsten plug 6 and the surface of the inter-layer insulator 3. The reduction in the level-difference or the sloped steps causes a broader and gentle waveform which is hard to be detected.
Meanwhile, the degree of miss-alignment of the resist pattern 8 with reference to the alignment hole 5 may be detected by comparing a first distance and a second distance, wherein the first distance is defined to be a distance between the waveform peak “B” representing one vertical wall of the resist pattern hole 8-a and the waveform peak “A” representing one sloped step of the box-mark 9, while the second distance is defined to be another distance between the waveform peak “C” representing opposite vertical wall of the resist pattern hole 8-a and the waveform peak “D” representing opposite sloped step of the box-mark 9. If the first and second distance are substantially the same, then this means that the alignment is almost perfect. The degree of the miss-alignment may read on the difference between the first and second distances.
The broader and gentle waveforms due to the reduction in the level-difference or the sloped steps of the box-mark 9 reduces the accuracy in measuring the difference between the first and second distances or in detecting the degree of the miss-alignment.
In the above circumstances, the development of a novel alignment pattern and a method of forming the same free from the above problems is desirable.